Ocean stratification

The density of water in the ocean, which is defined as mass per unit of volume, has a complicated dependence on temperature (T), salinity (S) and pressure (p), which in turn is a function of the density and depth of the overlying water, and is denoted as \rho(S, T, p). The dependence on pressure is not significant, since seawater is almost perfectly incompressible. A change in the temperature of the water impacts on the distance between water parcels directly. When the temperature of the water increases, the distance between water parcels will increase and hence the density will decrease. Salinity is a measure of the mass of dissolved solids, which consist mainly of salt. Increasing the salinity will increase the density. Just like the pycnocline defines the layer with a fast change in density, similar layers can be defined for a fast change in temperature and salinity: the thermocline and the halocline. Since the density depends on both the temperature and the salinity, the pycno-, thermo-, and haloclines have similar shapes. The difference is that the density increases with depth, whereas the salinity and temperature decrease with depth. In the ocean, a specific range of temperature and salinity occurs. Using the GODAS Data, \rho = \frac{\rho(S, T, 0)}{1-\frac{p}{K(S, T, p)}}.The terms in this formula, density when the pressure is zero, \rho(S, T, 0), and a term involving the compressibility of water, K(S, T, p), are both heavily dependent on the temperature and less dependent on the salinity:\begin{align}\rho(S, T, 0) = \rho_{SMOW} + B_1S + C_1S^{1.5} + d_0S^2, &\qquad K(S, T, p) =K(S, T, 0) + A_1p + B_2p^2, \end{align}with: \begin{align} {} & \rho_{SMOW} = a_0 + a_1T + a_2T^2 + a_3T^3 + a_4T^4 + a_5T^5, \\ {} & B_1 = b_0 + b_1T + b_2T^2 + b_3T^3 + b_4T^4, \\ {} & C_1 = c_0 + c_1T + c_2T^2, \\ \end{align} and \begin{align} {} & K(S, T, 0) = K_w + F_1S + G_1S^{1.5}, \\ {} & K_w = e_0 + e_1T + e_2T^2 + e_3T^3 + e^4T^4, \\ {} & F_1 = f_0 + f_1T + f_1T + f_2T^2 + f_3T^3, \\ {} & G_1 = g_0 + g_1T + g_2T^2, \\ {} & A_1 = A_w + (i_0 + i_1T + i_2T^2)S + j_0S^{1.5}, \\ {} & A_w = h_0 + h_1T + h_2T^2 + h_3T^3, \\ {} & B_2 = B_w + (m_0 + m_1T + m_2T^2)S), \\ {} & B_w = k_0 + k_1T + k_2T^2. \end{align} In these formulas, all of the small letters, a_i, b_i, c_i, d_0, e_i, f_i, g_i, i_i, j_0, h_i, m_i and k_i are constants that are defined in Appendix A of a book on Internal Gravity Waves, published in 2015. The density depends more on the temperature than on the salinity, as can be deduced from the exact formula and can be shown in plots using the GODAS Data. In the plots regarding surface temperature, salinity and density, it can be seen that locations with the coldest water, at the poles, are also the locations with the highest densities. The regions with the highest salinity, on the other hand, are not the regions with the highest density, meaning that temperature contributes mostly to the density in the oceans. A specific example is the Arabian Sea. ==Quantification==

Ocean stratification can be defined and quantified by the change in density with depth. The Buoyancy frequency, also called the Brunt-Väisälä frequency, can be used as direct representation of stratification in combination with observations on temperature and salinity. The Buoyancy frequency, N, represents the intrinsic frequency of internal gravity waves. In some parts of the ocean unstable stratification appears, leading to convection. If the stratification in a water column increases, implying an increase of the value N^2, turbulent mixing and hence the eddy viscosity will decrease. Furthermore, an increase of N^2, implies an increase of |\partial\rho/\partial z|, meaning that the difference in densities in this water column increase as well. Throughout the year, the oceanic stratification is not constant, since the stratification depends on density, and therefore on temperature and salinity. The interannual fluctuations in tropical Pacific Ocean stratification are dominated by El Niño, which can be linked with the strong variations in the thermocline depth in the eastern equatorial Pacific. On the other hand, mixing from tropical storms also tends to reduce stratification differences among layers. ==Observations on increasing stratification==

Temperature and salinity changes due to global warming and climate change alter the ocean density and lead to changes in vertical stratification. The regions with the currently deepest mixed layers are associated with the greatest mixed layer shoaling, particularly the North Atlantic and Southern Ocean basin. and NCAR & UCAR News ). In the figure below, the trends of the change in stratification in all of the ocean basins have been plotted. This data shows that over the years the stratification has drastically increased. The changes in stratification are greatest in the Southern Ocean, followed by the Pacific Ocean. In the Pacific Ocean, the increase of stratification in the eastern equatorial has found to be greater than in the western equatorial. This is likely to be linked to the weakening of the trade winds and reduced upwelling in the eastern Pacific, which can be explained by the weakening of the Walker circulation. ==Causes and consequences==

Temperature and mixing The change in temperature dominates the increasing stratification, while salinity only plays a role locally. Even though approximately 70% of the Earth's surface consists of water, more than 75% of the water exchange between the Earth's surface and the atmosphere occurs over the oceans. The ocean absorbs part of the energy from sunlight as heat and is initially absorbed by the surface. Eventually a part of this heat also spreads to deeper water. Greenhouse gases absorb extra energy from the sun, which is again absorbed by the oceans, leading to an increase in the amount of heat stored by the oceans. The increase of temperature of the oceans goes rather slow, compared to the atmosphere. However, the ocean heat uptake has doubled since 1993 and oceans have absorbed over 90% of the extra heat of the Earth since 1955. In the Arctic, the decrease of salinity, and hence density, can be explained by the input of freshwater from melting glaciers and ice sheets. This process and the increase of stratification in the Arctic will continue with the current carbon emissions. Since oxygen plays a direct and important role in the cycles of carbon, nitrogen and many other elements such as phosphorus, iron and magnesium, de-oxygenation will have large consequences. It plays a vital role for many organisms and the variety of ocean animals of all kinds. The de-oxygenation in subsurface waters is due to the decrease in ocean mixing, which is caused by the increase of stratification in the upper ocean. while in others the differing densities of stratified waters act to separate species from one another. Stratification of water limits the distribution of nutrients required for life, resulting in oligotrophic regions spreading as the surface thermocline strengthens. In many already stratified waters, such as subtropical gyres or equatorial waters, winter storms degrade the stratification and introduce much needed nutrients from the deep ocean. In a warming ocean, the energy required to breach the pycnocline will be greater requiring stronger mixing events to have the same effect. Similarly where cooler water upwells from deep currents, in a more strongly stratified ocean this upwelling will also require more energy and nutrients will be limited in reaching the surface in places. The microbial pump is likely to play a larger role in nutrient cycling in the stratified surface ocean with less biological activity occurring to move dissolved organic carbon to the deep ocean. This is due partly to decreased mixing and partly to warmer water holding less oxygen. ==Mixed layer depth (MLD)==

The surface mixed layer is the uppermost layer in the ocean and is well mixed by mechanical (wind) and thermal (convection) effects. Turbulence in this layer occurs through surface processes, for example wind stirring, surface heat fluxes and evaporation, The mixed layer is inherently the layer most connected to the atmosphere and affects and is affected by all weather systems, especially those with strong winds such as hurricanes. Throughout the year, the depth of the mixed layer varies. The thickness of the layer increases in wintertime and decreases in the summer. If the mixed layer is really deep, less light can reach the phytoplankton. Phytoplankton have been shown to be important in the global carbon cycle. Furthermore, since phytoplankton are at the bottom of the food chain, a decrease in phytoplankton can have consequences on a very large scale. An exact relation between an increase in stratification and a change in the mixed layer depth has not yet been determined and remains uncertain. Although some studies suggest that a thinner mixed layer should accompany a more stratified upper ocean, other work reports seasonal deepening of the mixed layer since 1970. There is literature substantiating the statement that in the years from 1970 to 2018, the stratification in the basis of the mixed layer as well as the depth of the mixed layer have increased. Contradicting this result, other literature states a decrease of the depth of the mixed layer partly as a result of the increase of upper-ocean stratification. It has been found that the mixed layer in the extension of the Kuroshio Current, at the west side of the North Pacific, has decreased more than 30 meters. This shoaling is caused by weakening of wind and a reduction of seasonal vertical mixing. Furthermore, there exists research stating that heating of the surface of the ocean, and hence an increase in stratification, does not necessarily mean an increase nor decrease in the mixed layer depth. Using the GODAS Data it can be seen that the depth of the mixed layer has increased as well as decreased over time. Between 1970 and 2018, the summertime mixed-layer depth (MLD) deepened by 2.9 ± 0.5% per decade (or 5 to 10 m per decade, depending on the region), and the Southern Ocean experienced the greatest deepening. However, there is limited observational evidence that the mixed layer is globally deepening, and only under strong greenhouse gas emissions scenarios do global mixed-layer depths shoal in the 21st century. Although it is virtually certain that upper ocean stratification will increase through the 21st century, scientists express low confidence in how the mixed-layer depth will evolve. ==See also==